Thermoplastic polyolefins (TPOs) have a wide range of applications, most notably in automobile parts. In general, TPOs are defined as impact modified polymers containing greater than 20 weight percent of ethylene/propylene rubber (EPR) or other rubber. It is known that TPOs may be produced either by mechanical compounding of a polypropylene homopolymer or copolymer with an EPR, other rubber, or copolymer with additional rubber. Alternatively, TPOs may be produced by forming the rubber phase in-reactor consecutively after forming the propylene homopolymer or copolymer. Reactor production is more cost effective and is known to provide more consistent properties, resulting in cost savings of up to 10–15%
It is known that the properties of TPOs are governed by various structural features, such as rubber particle size and distribution, isotacticity of the propylene homopolymer and the composition (e.g. ethylene content) of the rubber phase. Maier and Calafut report in “Polypropylene: The Definitive User's Guide and Handbook”, p. 23, ©1998, Plastic Design Library, in-reactor made TPOs have rubber phases with finer particle sizes that are more uniformly distributed in the polypropylene matrix, but that properties cannot be tailored for specific applications. It is also known that it is difficult to obtain desirable properties in TPOs that are made in-reactor, especially when the material has a melt flow rate of 20 dg/min or greater at 230° C. These properties include stiffness/impact balance, tensile elongation at break and surface gloss. In order to improve these properties of in-reactor materials, controlled rheology (CR) is often employed, in which a low melt flow rate TPO is cracked (visbroken) to a higher melt flow rate in the presence of peroxide, such as to greater than 20 dg/min at 230° C. Although controlled rheology improves tensile elongation at break, low temperature impact properties, and surface gloss in molded articles, the stiffness of such articles is reduced significantly compared to non-CR'd (non-visbroken) materials.
The generally accepted conditions for optimized impact properties in impact modified polymers are uniform rubber particle dispersion and a small, optimum, rubber particle size. In, “Development of UNIPOL™ PP High Melt Flow Polypropylene Impact Copolymers with Improved Impact Stiffness Balance”, 1999, presented at PROPYLENE '99, Kersting reports that the melt viscosity ratio between the EPR rubber phase and the propylene homopolymer phase is the controlling factor for impact properties. Kersting further reports that for a medium impact copolymer having 14–20 weight percent of a rubber phase containing 50–55 weight percent ethylene, the impact/stiffness balance is significantly improved at a high viscosity ratio. The high ratio reported is indicative of a high molecular weight EPR.
In contrast, U.S. Pat. No. 5,258,464 to McCullough, Jr. et al. teaches polypropylene impact copolymers that have an improved resistance to stress whitening. In the impact copolymers disclosed by McCullough, Jr., the ratio of the intrinsic viscosities of the propylene homopolymer phase and the copolymer rubber phase is near 1 as determined by an equation provided in the specification of McCullough, Jr. The copolymer phase of the impact copolymers disclosed in McCullough, Jr. comprises 10 to 50 weight percent of the total polymer. Further, the copolymer phase contains from 38 to 60 weight percent of ethylene based on the total weight of the copolymer phase. According to the teaching of McCullough, Jr., the actual intrinsic viscosities of the homopolymer and copolymer phase are not important, so long as the ratio of those viscosities is close to 1. McCullough, Jr. also teaches that when polymers of high melt flow are prepared the required ratio of intrinsic viscosities can only be obtained in a gas phase polymerization.
Both Kersting and McCullough, Jr. rely on viscosity ratios as a measure of molecular weight ratio for optimizing the performance of impact modified polypropylene. However, Kersting does not address TPOs and McCullough does not address the balancing of impact strength and stiffness. Additionally, both melt viscosity and intrinsic viscosity, while related to molecular weight, are imprecise measures of molecular weight.
As a result, there remains a need for high melt flow in reactor produced TPOs that combine the desirable properties of an in reactor TPO and a compounded TPO, without the need for visbreaking. Further, there remains a need for such high melt flow, in reactor produced TPOs that can be produced in a liquid phase, gas phase or combined liquid/gas phase polymerization.